1. Field of the Invention
The present invention is related to the field of electric wireline oil well servicing tools. More particularly, the present invention is related to the use of electric wireline tools which are lowered into a wellbore by means of coiled tubing. Coiled tubing is typically used to transport well servicing tools, including electric wireline tools, to the bottom of a wellbore which has been drilled nearly horizontally. In these cases, gravity cannot transport the tools to the bottom of the wellbore, so conveyances such as coiled tubing are necessary.
2. Discussion of the Related Art
Conveyance of well servicing tools to the bottom of highly deviated wellbores with drillpipe or workover rig tubing is known in the art. For example, "Extended Reach and Horizontal Well Services", Western Atlas International Houston, Tex. 1990, describes some of these methods. Drillpipe or workover tubing is formed from thirty foot long sections with threaded connections at each end. The threaded connections join the ends of each section to form a continuous length of pipe. The time consumed in threading these connections together is considerable.
Use of coiled tubing in drilling and workover operations, for replacing the step of assembling the sections of drillpipe or tubing, is also known in the art. U.S. Pat. No. 3,116,793 teaches a method of using coiled tubing to pump completion and workover fluids to a desired depth in a wellbore. U.S. Pat. No. 4,850,440 teaches a method of creating a wellbore with a coiled tubing conveyed drilling apparatus. U.S. Pat. No. 3,285,629 teaches a method of conveyance of electrically powered tools, which could include electric wireline tools, by a continuous hose having a nested electrical cable. The invention disclosed in U.S. Pat. No. 3,285,629 is a drilling tool which is conveyed to the desired depth by gravity. The weight of the drilling tool itself provides the conveyance force required. Consequently, conveyance of tools using the hose disclosed in U.S. Pat. No. 3,285,629, to the bottom of a highly deviated or horizontal wellbore would be difficult since the force of gravity would be largely dissipated by friction with the bottom wall of the wellbore. Additionally, the electrical cable described in this patent is assembled into a continuous length from discrete sections joined by connectors. The use of these connectors would add significantly to the cost of the electric wireline.
The use of electric wireline tools conveyed by coiled tubing into deviated wells is also known in the art. "World Oil's Coiled Tubing Handbook", Gulf Publishing Co., Houston, Tex. 1993, describes such operations in detail. The use of coiled tubing to convey electric wireline tools requires that the electric wireline be inserted coaxially through the entire length of coiled tubing. "World Oil's Coiled Tubing Handbook", referenced above, discloses a method for installing electric wireline inside coiled tubing. The coiled tubing is unspooled into a generally straight shape and the electric wireline is then pumped through the unspooled tubing. This operation requires either the use of a wellbore deep enough to accommodate the unspooled length of tubing, or unspooling the tubing along the ground. Obtaining the use of a wellbore with sufficient depth is difficult and expensive. Unspooling the tubing along the ground in a roughly straight line is difficult since the length of the tubing often exceeds three miles. In either of these methods, the tubing must undergo an additional unspooling to install the wireline. Additional unspooling of the tubing shortens its useful working lifespan because of the bending stress applied to the tubing as a result of the spooling process.
Insertion of the electric wireline inside the coiled tubing while the tubing is still spooled is difficult because of the frictional forces developed between the interior wall of the tubing and the wireline as the wireline insertion progresses. The wireline tends to rest upon the inside radius of the wall of the coiled tubing. As insertion of the wireline proceeds, the total contact area between the wireline and the tubing increases. The contact area between wireline and coiled tubing would be the same as in the case of unspooled tubing, but the tubing is wound in a roughly circular pattern so a partial component of the tension applied to the cable during insertion acts in a direction normal to the axis of the wireline. This normal force is similar in effect to the application of weight upon wireline travelling through unspooled tubing. The friction between cable and tubing is proportionately increased with each succeeding wrap around the spool by the wireline. The result is that substantially before the wireline reaches the end of the coiled tubing, the frictional forces developed would exceed the safe tension loading of the wireline and the installation would fail.